A disk storage or disc storage is a general category of storage mechanisms, in which data are digitally recorded by various electronic, magnetic, optical, or mechanical methods on a surface layer deposited of one or more planar, round and rotating disks (or discs) (also referred to as the media). A disk drive is a device implementing such a storage mechanism with fixed or removable media; with removable media the device is usually distinguished from the media as in compact disc drive and the compact disc. Notable types are the hard disk drives (HDD) containing a non-removable disk, the floppy disk drive (FDD) and its removable floppy disk, and various optical disc drives and associated optical disc media (www.wikipedia.org).
RAID (redundant array of independent disks) is a storage technology that combines multiple disk drive components into a logical unit. Data is distributed across the drives in one of several ways called “RAID levels”, depending on the level of redundancy and performance required (www.wikipedia.org).
RAID is used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple physical drives: RAID is an example of storage virtualization and the array can be accessed by the operating system as one single drive.
The different schemes or architectures are named by the word RAID followed by a number (e.g., RAID 0, RAID 1, RAID 2, RAID 3, RAID 4, RAID 5, RAID 6 and RAID 10). Each scheme provides a different balance between the key goals: reliability and availability, performance and capacity. RAID levels greater than RAID 0 provide protection against unrecoverable (sector) read errors, as well as whole disk failure.
A number of standard schemes have evolved which are referred to as levels. There were five RAID levels originally conceived, but many more variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary). RAID levels and their associated data formats are standardized by the Storage Networking Industry Association (SNIA) in the Common RAID Disk Drive Format (DDF) standard.
RAID 5, 6 and 10 levels are commonly used in the industry.
RAID 5 (block-level striping with distributed parity) distributes parity along with the data and requires all drives but one to be present to operate. The array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. RAID 5 requires at least three disks.
RAID 6 (block-level striping with double distributed parity) provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems. This becomes increasingly important as large-capacity drives lengthen the time needed to recover from the failure of a single drive. Like RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced and the associated data rebuilt.
In RAID 10 (often referred to as RAID 1+0) (mirroring and striping), data are written in stripes across primary disks that have been mirrored to the secondary disks.
Modern storage systems may include large numbers of disk drives. There is a growing need to provide reliable and efficient storage systems.